It seems that you don't use TeX Live 2018 but the 2017 version. Online updates/installations are only possible with the current version, that's 2018. Verify that it's 2018 or install 2018. Perhaps it could work anyway if you run

sh update-tlmgr-latest.sh -- --upgrade

at the directory where that .sh script is, probably in /usr/local/texlive/2017. Or